derivepassphrase.git

1) # SPDX-FileCopyrightText: 2024 Marco Ricci <m@the13thletter.info>
2) #
3) # SPDX-License-Identifier: MIT
4) 
5) """A bare-bones SSH agent client supporting signing and key listing."""
6) 
7) from __future__ import annotations
8) 
9) import collections
10) import errno
11) import os
12) import socket

13) from typing import TYPE_CHECKING

14) 

15) from typing_extensions import Self

16) 

17) from ssh_agent_client import types as ssh_types
18) 
19) if TYPE_CHECKING:
20)     import types
21)     from collections.abc import Sequence

22) 
23) __all__ = ('SSHAgentClient',)

24) __author__ = "Marco Ricci <m@the13thletter.info>"

25) __version__ = "0.1.1"

26) 

27) # In SSH bytestrings, the "length" of the byte string is stored as
28) # a 4-byte/32-bit unsigned integer at the beginning.
29) HEAD_LEN = 4
30) 

31) _socket = socket
32) 

33) class TrailingDataError(RuntimeError):
34)     """The result contained trailing data."""

35)     def __init__(self):
36)         super().__init__('Overlong response from SSH agent')

37) 

38) class SSHAgentClient:
39)     """A bare-bones SSH agent client supporting signing and key listing.
40) 
41)     The main use case is requesting the agent sign some data, after
42)     checking that the necessary key is already loaded.
43) 
44)     The main fleshed out methods are `list_keys` and `sign`, which
45)     implement the `REQUEST_IDENTITIES` and `SIGN_REQUEST` requests.  If
46)     you *really* wanted to, there is enough infrastructure in place to
47)     issue other requests as defined in the protocol---it's merely the
48)     wrapper functions and the protocol numbers table that are missing.
49) 
50)     """

51)     _connection: socket.socket

52)     def __init__(
53)         self, /, *, socket: socket.socket | None = None, timeout: int = 125
54)     ) -> None:
55)         """Initialize the client.
56) 
57)         Args:
58)             socket:
59)                 An optional socket, connected to the SSH agent.  If not
60)                 given, we query the `SSH_AUTH_SOCK` environment
61)                 variable to auto-discover the correct socket address.
62)             timeout:
63)                 A connection timeout for the SSH agent.  Only used if
64)                 the socket is not yet connected.  The default value
65)                 gives ample time for agent connections forwarded via
66)                 SSH on high-latency networks (e.g. Tor).
67) 

68)         Raises:
69)             KeyError:
70)                 The `SSH_AUTH_SOCK` environment was not found.
71)             OSError:
72)                 There was an error setting up a socket connection to the
73)                 agent.
74) 

75)         """
76)         if socket is not None:

77)             self._connection = socket

78)         else:

79)             self._connection = _socket.socket(family=_socket.AF_UNIX)

80)         try:

81)             # Test whether the socket is connected.
82)             self._connection.getpeername()

83)         except OSError as e:

84)             # This condition is hard to test purposefully, so exclude
85)             # from coverage.
86)             if e.errno != errno.ENOTCONN:  # pragma: no cover

87)                 raise

88)             if 'SSH_AUTH_SOCK' not in os.environ:

89)                 msg = 'SSH_AUTH_SOCK environment variable'
90)                 raise KeyError(msg) from None

91)             ssh_auth_sock = os.environ['SSH_AUTH_SOCK']

92)             self._connection.settimeout(timeout)

93)             self._connection.connect(ssh_auth_sock)

94) 
95)     def __enter__(self) -> Self:

96)         """Close socket connection upon context manager completion."""
97)         self._connection.__enter__()

98)         return self
99) 
100)     def __exit__(

101)         self,
102)         exc_type: type[BaseException] | None,
103)         exc_val: BaseException | None,
104)         exc_tb: types.TracebackType | None,

105)     ) -> bool:

106)         """Close socket connection upon context manager completion."""

107)         return bool(

108)             self._connection.__exit__(

109)                 exc_type, exc_val, exc_tb)  # type: ignore[func-returns-value]
110)         )
111) 
112)     @staticmethod

113)     def uint32(num: int, /) -> bytes:
114)         r"""Format the number as a `uint32`, as per the agent protocol.
115) 
116)         Args:
117)             num: A number.
118) 
119)         Returns:
120)             The number in SSH agent wire protocol format, i.e. as
121)             a 32-bit big endian number.
122) 
123)         Raises:
124)             OverflowError:
125)                 As per [`int.to_bytes`][].
126) 
127)         Examples:
128)             >>> SSHAgentClient.uint32(16777216)
129)             b'\x01\x00\x00\x00'
130) 
131)         """

132)         return int.to_bytes(num, 4, 'big', signed=False)
133) 
134)     @classmethod
135)     def string(cls, payload: bytes | bytearray, /) -> bytes | bytearray:

136)         r"""Format the payload as an SSH string, as per the agent protocol.
137) 
138)         Args:
139)             payload: A byte string.
140) 
141)         Returns:
142)             The payload, framed in the SSH agent wire protocol format.
143) 
144)         Examples:
145)             >>> bytes(SSHAgentClient.string(b'ssh-rsa'))
146)             b'\x00\x00\x00\x07ssh-rsa'
147) 
148)         """

149)         try:
150)             ret = bytearray()
151)             ret.extend(cls.uint32(len(payload)))
152)             ret.extend(payload)
153)         except Exception as e:

154)             msg = 'invalid payload type'
155)             raise TypeError(msg) from e
156)         return ret

157) 
158)     @classmethod
159)     def unstring(cls, bytestring: bytes | bytearray, /) -> bytes | bytearray:

160)         r"""Unpack an SSH string.
161) 
162)         Args:
163)             bytestring: A framed byte string.
164) 
165)         Returns:
166)             The unframed byte string, i.e., the payload.
167) 
168)         Raises:
169)             ValueError:

170)                 The byte string is not an SSH string.

171) 
172)         Examples:
173)             >>> bytes(SSHAgentClient.unstring(b'\x00\x00\x00\x07ssh-rsa'))
174)             b'ssh-rsa'

175)             >>> bytes(SSHAgentClient.unstring(
176)             ...     SSHAgentClient.string(b'ssh-ed25519')))

177)             b'ssh-ed25519'
178) 
179)         """

180)         n = len(bytestring)

181)         msg = 'malformed SSH byte string'
182)         if (
183)             n < HEAD_LEN
184)             or n != HEAD_LEN + int.from_bytes(bytestring[:HEAD_LEN], 'big',
185)                                               signed=False)
186)         ):
187)             raise ValueError(msg)
188)         return bytestring[HEAD_LEN:]

189) 

190)     @classmethod
191)     def unstring_prefix(
192)         cls, bytestring: bytes | bytearray, /
193)     ) -> tuple[bytes | bytearray, bytes | bytearray]:
194)         r"""Unpack an SSH string at the beginning of the byte string.
195) 
196)         Args:
197)             bytestring:
198)                 A (general) byte string, beginning with a framed/SSH
199)                 byte string.
200) 
201)         Returns:
202)             A 2-tuple `(a, b)`, where `a` is the unframed byte
203)             string/payload at the beginning of input byte string, and
204)             `b` is the remainder of the input byte string.
205) 
206)         Raises:
207)             ValueError:
208)                 The byte string does not begin with an SSH string.
209) 
210)         Examples:
211)             >>> a, b = SSHAgentClient.unstring_prefix(
212)             ...     b'\x00\x00\x00\x07ssh-rsa____trailing data')
213)             >>> (bytes(a), bytes(b))
214)             (b'ssh-rsa', b'____trailing data')
215)             >>> a, b = SSHAgentClient.unstring_prefix(
216)             ...     SSHAgentClient.string(b'ssh-ed25519'))
217)             >>> (bytes(a), bytes(b))
218)             (b'ssh-ed25519', b'')
219) 
220)         """
221)         n = len(bytestring)

222)         msg = 'malformed SSH byte string'
223)         if n < HEAD_LEN:
224)             raise ValueError(msg)
225)         m = int.from_bytes(bytestring[:HEAD_LEN], 'big', signed=False)
226)         if m + HEAD_LEN > n:
227)             raise ValueError(msg)
228)         return (bytestring[HEAD_LEN:m + HEAD_LEN], bytestring[m + HEAD_LEN:])

229) 

230)     def request(
231)         self, code: int, payload: bytes | bytearray, /
232)     ) -> tuple[int, bytes | bytearray]:
233)         """Issue a generic request to the SSH agent.
234) 
235)         Args:
236)             code:
237)                 The request code.  See the SSH agent protocol for
238)                 protocol numbers to use here (and which protocol numbers
239)                 to expect in a response).
240)             payload:
241)                 A byte string containing the payload, or "contents", of
242)                 the request.  Request-specific.  `request` will add any
243)                 necessary wire framing around the request code and the
244)                 payload.
245) 
246)         Returns:
247)             A 2-tuple consisting of the response code and the payload,
248)             with all wire framing removed.
249) 
250)         Raises:
251)             EOFError:
252)                 The response from the SSH agent is truncated or missing.
253) 
254)         """
255)         request_message = bytearray([code])
256)         request_message.extend(payload)

257)         self._connection.sendall(self.string(request_message))

258)         chunk = self._connection.recv(HEAD_LEN)
259)         if len(chunk) < HEAD_LEN:
260)             msg = 'cannot read response length'
261)             raise EOFError(msg)

262)         response_length = int.from_bytes(chunk, 'big', signed=False)

263)         response = self._connection.recv(response_length)

264)         if len(response) < response_length:

265)             msg = 'truncated response from SSH agent'
266)             raise EOFError(msg)

267)         return response[0], response[1:]
268) 

269)     def list_keys(self) -> Sequence[ssh_types.KeyCommentPair]:

270)         """Request a list of keys known to the SSH agent.
271) 
272)         Returns:
273)             A read-only sequence of key/comment pairs.
274) 
275)         Raises:
276)             EOFError:
277)                 The response from the SSH agent is truncated or missing.

278)             TrailingDataError:

279)                 The response from the SSH agent is too long.

280)             RuntimeError:
281)                 The agent failed to complete the request.

282) 
283)         """
284)         response_code, response = self.request(

285)             ssh_types.SSH_AGENTC.REQUEST_IDENTITIES.value, b'')
286)         if response_code != ssh_types.SSH_AGENT.IDENTITIES_ANSWER.value:
287)             msg = (f'error return from SSH agent: '
288)                    f'{response_code = }, {response = }')
289)             raise RuntimeError(msg)

290)         response_stream = collections.deque(response)
291)         def shift(num: int) -> bytes:

292)             buf = collections.deque(b'')
293)             for _ in range(num):

294)                 try:
295)                     val = response_stream.popleft()
296)                 except IndexError:
297)                     response_stream.extendleft(reversed(buf))

298)                     msg = 'truncated response from SSH agent'
299)                     raise EOFError(msg) from None

300)                 buf.append(val)
301)             return bytes(buf)
302)         key_count = int.from_bytes(shift(4), 'big')

303)         keys: collections.deque[ssh_types.KeyCommentPair]

304)         keys = collections.deque()

305)         for _ in range(key_count):

306)             key_size = int.from_bytes(shift(4), 'big')
307)             key = shift(key_size)
308)             comment_size = int.from_bytes(shift(4), 'big')
309)             comment = shift(comment_size)
310)             # Both `key` and `comment` are not wrapped as SSH strings.

311)             keys.append(ssh_types.KeyCommentPair(key, comment))

312)         if response_stream:

313)             raise TrailingDataError

314)         return keys
315) 
316)     def sign(
317)         self, /, key: bytes | bytearray, payload: bytes | bytearray,
318)         *, flags: int = 0, check_if_key_loaded: bool = False,
319)     ) -> bytes | bytearray:
320)         """Request the SSH agent sign the payload with the key.
321) 
322)         Args:
323)             key:
324)                 The public SSH key to sign the payload with, in the same
325)                 format as returned by, e.g., the `list_keys` method.
326)                 The corresponding private key must have previously been
327)                 loaded into the agent to successfully issue a signature.
328)             payload:
329)                 A byte string of data to sign.
330)             flags:
331)                 Optional flags for the signing request.  Currently
332)                 passed on as-is to the agent.  In real-world usage, this
333)                 could be used, e.g., to request more modern hash
334)                 algorithms when signing with RSA keys.  (No such
335)                 real-world usage is currently implemented.)
336)             check_if_key_loaded:
337)                 If true, check beforehand (via `list_keys`) if the
338)                 corresponding key has been loaded into the agent.
339) 
340)         Returns:
341)             The binary signature of the payload under the given key.
342) 
343)         Raises:
344)             EOFError:
345)                 The response from the SSH agent is truncated or missing.

346)             TrailingDataError:

347)                 The response from the SSH agent is too long.
348)             RuntimeError:
349)                 The agent failed to complete the request.

350)             KeyError:

351)                 `check_if_key_loaded` is true, and the `key` was not
352)                 loaded into the agent.
353) 
354)         """
355)         if check_if_key_loaded:
356)             loaded_keys = frozenset({pair.key for pair in self.list_keys()})
357)             if bytes(key) not in loaded_keys:

358)                 msg = 'target SSH key not loaded into agent'
359)                 raise KeyError(msg)

360)         request_data = bytearray(self.string(key))
361)         request_data.extend(self.string(payload))
362)         request_data.extend(self.uint32(flags))
363)         response_code, response = self.request(

364)             ssh_types.SSH_AGENTC.SIGN_REQUEST.value, request_data)
365)         if response_code != ssh_types.SSH_AGENT.SIGN_RESPONSE.value:
366)             msg = f'signing data failed: {response_code = }, {response = }'
367)             raise RuntimeError(msg)